Retail businesses for servicing vehicles with fuel requirements are commonly constructed with a plurality of underground storage tanks. Typically, such a "service station" may have one or more underground storage tanks for "leaded" and "unleaded" gasoline and in these categories multiple underground storage tanks for different octane ratings. In addition, a service station may have one or more underground diesel fuel tanks. Also, it is not uncommon for such service stations to have an underground storage tank for kerosene. To operate such stations as a profitable business it becomes important for the proprietor to know the amount of inventory of each fuel stored in the underground tanks. Considering that present underground storage tanks may hold several thousand gallons of fuel, the proprietor has a considerable investment in inventory at any one time.
Also for purposes of insuring continuing availability of each grade and octane rating of fuel it becomes important to periodically measure the volume of fuel in each underground tank for reordering purposes. With the continued proliferation of self-service stations which "pump" many gallons of fuel per day through automatic vending devices, it becomes more important for a proprietor of such a station to know the volume of fuel in each of the underground storage tanks.
One of the original and simplest techniques for measuring the volume of fuel in an underground storage tank is by use of long graduated measuring stick. An operator opened an access port in the top of each tank and lowered the measuring stick to the bottom and observed the wet-dry (liquid level) interface on the measuring stick. This number was then used to manually calculate the volume of fuel in the tank.
There has been a considerable advance since the measuring stick in techniques available for determining the volume of fuel in an underground tank. One such system is described in U.S. Pat. No. 4,349,882. This system utilizes an elongated multiple segment capacitance probe mounted in each storage tank. The liquid level and the volume of fuel in a tank is periodically measured by a microcomputer connected to the capacitance probe. U.S. Pat. No. 3,777,257 also describes apparatus that utilizes a capacitance probe for measuring the volume of fuel within a tank. These two patents are representative of systems available that replace the measuring stick for determining the volume of fuel in an underground storage tank.
Each of the presently known measuring systems that utilize a capacitance probe rely on the dielectric difference of the fuel as compared with the dielectric of air. It has been determined that the capacitance of the probe varies linearly as the probe becomes more fully submerged by fuel. This is based on the change in capacitance when the probe is exposed along its entire length to air as when the probe is completely submerged in a fuel. However, to accurately measure the height of liquid (fuel) and from the height measurement, calculate the volume, the dielectric constant of the liquid within the tank must be known and used in the computation of height or volume. Systems heretofore available relied on preset capacitor calibrations utilizing published values of dielectric constants for the liquid that was thought to be stored in a tank. This reliance on published dielectric constants results in inaccurate volume and height measurements.
In accordance with the present invention, the dielectric constant of the liquid within a tank is determined and then utilized in computing the liquid height measurement and in turn calculating the volume of fluid within the tank. The dielectric constant of the liquid within the tank is determined by measuring the capacitance value of a capacitor completely submerged by liquid. This capacitance value is then used in calculating liquid height and in turn computing volume.